This invention relates to multi-cell detectors for ionizing radiation such as x-radiation. The improved detector is generally applicable to detecting photon intensity distribution in a broad beam of x-rays and it is especially useful in x-ray computerized axial tomography systems.
In the computerized axial tomography process, a spatial distribution of x-ray photon intensities emerging from a body under examination is translated into discrete analog electric signals which are processed in a manner that enables reconstructing the x-ray image and displaying it as a visible image. Background information on the process is given in an article by Gordon et al, "Image Reconstruction From Projections", Scientific American, October 1975, Vol. 233, No. 4.
In some tomography systems, the x-ray beam is fan-shaped and diverges as it exits from the body being examined whereupon the beam falls on an array of detector cells such that photon intensities over the front of the beam can be detected and resolved spatially. Each active detector cell comprises at least a pair of electrode elements such as a pair of parallel thin metal plates. The x-ray source and detector orbit around an examination subject jointly. The individual detector cells are arranged in an array so that the x-ray photons distributed across the beam at any instant are detected simultaneously. The signals correspond with x-ray absorption along each ray path at the instant of detection. Additional sets of signals are obtained for a sequence of angular positions of the orbiting detector and x-ray source. The discrete analog signals are converted to digital signals and processed in a computer which is controlled by a suitable algorithm to produce signals representative of the x-ray absorption or attenuation of each small volume element in the body through which the x-ray beam passes. The analog signals are generally in the low nanoampere range. Careful attention must be given to maintaining an adequate signal-to-noise ratio.
A typical x-ray detector for use in a computerized axial tomography system that employs a broad front fan-shaped beam will usually require 300 or more individual detector cells to get adequate resolution. Hence, a conductor must be provided for each cell for conducting simultaneously produced signals from the inside of the detector housing to the preamplifiers of the data acquisition system electronic circuitry.
One prior method of conducting the analog signals from the individual cells uses insulating electric feed-throughs set in the cover of the detector housing. Each signal producing electrode comprising a cell has a fine lead wire spot welded to it and extending from it. Hundreds of solder connections had to be made as a result of having to run individual wires or a ribbon cable between each of the fine lead wires and the feed-throughs in the cover while the cover was held proximate to the electrode array. The wires extending from the fine leads on the electrode plates to the feed-throughs had to be long enough to provide sufficient clearance for making the solder connections at both ends. After the connections are made, the leads between the electrodes and the feed-throughs are folded into the electrode array housing and the cover is bolted onto the housing to effect a gas-tight seal. Another set of conductors are then connected to the outsides of the feed-throughs for sending the signals to the data acquisition and processing system.
One disadvantage of the approach just outlined is that the long leads between the electrodes and feed-throughs inside the housing had to be flexible and, hence, were subject to vibrations when the detector was used in x-ray tomography apparatus. Vibrations increase production of electric noise. Another disadvantage is that one end of each lead wire had to be soldered to one of the feed-throughs and the other end had to be soldered to the fine lead wires from the electrodes while the cover was held in spaced relationship with respect to the electrode array and before the cover could be applied to the detector housing. The soldering had to take place under very inconvenient circumstances.